Oxide superconductors have low carrier density. Namely the densities of carriers, i.e. electrons or holes which contribute to electric conduction, are inherently lower in oxide superconductors than in metal superconductors. All oxide superconductive films which have been reported hereto now were polycrystals. A polycrystal is an assembly of small single crystal granules with grain boundaries between neighboring granules. The grain boundaries have lattice defects. If the defects of the grain boundaries produce potential barriers, the barriers impair the electric conduction to a great extent. Thus polycrystal oxide superconductors have been useless for practical purposes as a material for producing electronic devices.
Therefore single crystals of oxide superconductive materials are indispensable for exploiting oxide superconductive materials for electrical use. Intensive research has been done for growing single crystal oxide superconductor films. However, no practical technology has matured for growing an oxide superconductor single crystal film of high quality. Furthermore, no big oxide superconductive bulk single crystal of good quality has been grown yet. Thus there has been no probability of the homoepitaxial growth of an oxide superconductive film upon an oxide superconductive single crystal substrate. In general, homoepitaxy is the best way for fabricating a high quality single crystal film of any material. However, homoepitaxy has been inapplicable to oxide superconductor material because of the lack of success in growing a big bulk single crystal.
Researchers have not succeeded yet in finding an acceptable substrate which enables an oxide superconductor to form a single crystal film of arbitrary orientation. Thus they produce substrates of oxide single crystals, for example, MgO or SrTiO.sub.3 and grow an oxide superconductor film upon the MgO or SrTiO.sub.3 single crystal substrate by heteroepitaxy. This is the present state of affairs for the fabrication of oxide superconductor films.
Cuprate superconductors are favored because they possess a high critical temperature Tc. The critical temperature means the transition temperature at which the state of the crystal changes from a superconductive condition to a normal conductive condition and vice versa. The material is superconductive below Tc but normal conductive above Tc. The cuprate superconductor has a layered crystal structure without cubic symmetry.
The lack of cubic symmetry differentiates the atomic arrangement in the planes parallel with the layers from the atomic arrangement in the direction normal to the layers. Anisotropy accompanies the structure of the layered arrangement of various atoms in the cuprate superconductor materials. The layered lattice structure promotes a strong anisotropy of the electric property in the cuprate superconductors. The controlling of the crystallographical direction (orientation) in growing the films is a basically important technology for allotting the desired electric property to the film crystal, especially when superconductive devices are produced on the grown single crystal films.
A junction is a fundamental element in the fabrication of electronic devices from superconductive materials. Junctions are made from multilayer film in many cases. Film crystals are important for the fabrication of junctions. In the case of superconductors, a superconductive coherent length is one of the most important parameters of the junction. The properties of superconductivity are greatly sensitive to the fluctuation of a crystallographical structure or the fluctuation of components in the oxide superconductors. If the interface of the junction has defects, the defects cause a degenerate layer to form. In this case, a short coherent length impedes the generation of a good junction property.
As explained before, the present crystal growth technology involves the heteroepitaxy of the oxide superconductive film on a single crystal MgO substrate or a single crystal SrTiO.sub.3 substrate which can be easily grown by the current technology. Nevertheless, these commonplace bulk crystals have a lattice structure different from the layered structure of the oxide superconductor (orthorhombic symmetry). The crystals (MgO or SrTiO.sub.3) for substrates have a cubic symmetry. Thus there is no crystallographical difference between c-axis and a-axis (a-axis also represents b-axis, because the a-axis and b-axis are equivalent or nearly equivalent axes). All a-, b- and c-axes are equivalent on crystallography in MgO and SrTiO.sub.3 crystals.
The lattice structure is explained first. FIG. 8 is an exploded perspective representation of the unit lattice structure of an MgO crystal. An Mg atom has six nearest neighbor oxygen atoms in six directions perpendicular with each other. An oxygen atom has also six nearest neighbor Mg atoms in six perpendicular directions. However, there is no difference between a-axis, b-axis and c-axis. This is a very simple cubic crystal lattice. No anisotropy occurs between a-axis (including b-axis) and c-axis. MgO has a cubic lattice structure.
FIG. 9 is an exploded perspective representation of the unit lattice structure of SrTiO.sub.3. Each of the six square planes has an oxygen atom at the center. Sr atoms occupy eight corners of the lattice. A Ti atom lies at the center of the lattice. The lattice is entirely symmetric in three directions. A-axis, b-axis and c-axis are equivalent. The lattice has no anisotropy. This is also a cubic lattice structure.
FIG. 10 is an exploded perspective representation of the unit lattice structure of YBa.sub.2 Cu.sub.3 O.sub.7-x (abbreviated YBCO). The lattice has a square pillar (orthorhombic) structure. Two Ba atoms vertically align at the center line. A Y atom interposes between the two Ba atoms. O atoms are arranged along the side rectangular planes of the pillar. Four sets of four Cu atoms are aligned along the four vertical lines of the lattice pillar. The lattice has three layers in the vertical direction. A single layer has four Cu atoms. A Cu atom has four O atoms as the nearest neighbor at every 90 degrees around on some layer. Blank rounds and dotted rounds mean O atoms in the figure. The lattice structure is nearly equivalent in a-axis and b-axis. But the arrangement of atoms in c-axis is totally different from the arrangement in a-axis or b-axis. C-axis is longer than a-axis or b-axis. C-axis is unequivalent to a-axis or b-axis. There is a strong anisotropy between a-axis and c-axis in YBCO.
When a cuprate superconductor (e.g. YBCO) film is grown on a MgO substrate or a cubic perovskite substrate (e.g. SrTiO.sub.3), the film and the substrate have different lattice structures. The substrate is isotropic in all three directions. But the film has anisotropy with regard to c-axis. The film has different properties along c-axis from the properties in a-axis or b-axis direction.
Homoepitaxy can determine the crystal orientation of a film uniquely by the crystal orientation of the substrate. The orientation of the film is the same as the orientation of the substrate.
Unlike homoepitaxy, heteroepitaxy cannot determine the orientation of the crystal structure of the film by the orientation of the substrate because of the difference in the lattice structures. Now two different films are defined for the growing superconductive films. "A-axis orientated film" is a film whose normal is parallel with a-axis. Namely the surface of the film is parallel both with c-axis and with b-axis. "C-axis orientated film" has a normal parallel with c-axis.
Because of anisotropy, a-axis orientated films may be expected to be produced with the same probability as c-axis oriented films, when cuprate superconductor films are heteroepitaxially grown on the cubic single crystal substrates, e.g. MgO or SrTiO.sub.3. But this does not in fact hold true. The temperature of the substrate determines whether the seed crystal yields a c-axis oriented film or an a-axis oriented film. Present heteroepitaxy only makes either a-axis oriented films with many defects or c-axis oriented films of oxide superconductors on the commonplace cubic crystals.
An oxide superconductor has another anisotropy besides the lattice anisotropy explained above. This is the anisotropy of growing speed. The growing velocity is faster in a-axis and b-axis than in c-axis. In general, the axes which are flavored with the fastest growing velocity are likely to become parallel with the surface of the growing film. Since a-axis and b-axis have faster speed of growth, the surface of a growing film tends to include both a-axis and b-axis. Thus films are apt to become c-axis oriented films. Therefore the orientation of a growing film is determined not by the lattice orientation of a substrate but by the structural anisotropy of the oxide superconductor itself in the heteroepitaxy of oxide superconductive materials.
For example, the heteroepitaxy on a cubic perovskite substrate is likely to produce a film which has a normal parallel with c-axis. C-axis oriented films are mainly produced by the heteroepitaxy on a cubic perovskite substrate (e.g. SrTiO.sub.3). If a plurality of c-axis oriented films are deposited on a substrate, the junctions between the neighboring films are normal to c-axis. Such a junction whose normal is parallel with c-axis is called a "c-axis junction".
If the coherent length along c-axis is short, a current flows across the c-axis junction with difficulty. The short coherent length deteriorates the property of the junction.
.xi. denotes the coherent length which denotes the extension of a Cooper pair. The coherent length is defined with regard to a certain direction. .xi..sub.a is the coherent length along a-axis. .xi..sub.o is the coherent length in the direction of c-axis. YBCO has an a-axis coherent length .xi..sub.a far longer than a c-axis coherent length .xi..sub.o. Namely .xi..sub.a &gt;&gt;.xi..sub.o.
If a coherent length in a certain direction is long, a large current can flow through a junction which is perpendicular to the direction. Since the direction of a-axis (or b-axis) has a long coherent length, an a-axis junction (which is perpendicular to a-axis) enables a large current to flow across the junction. Therefore the a-axis oriented film is far superior in the current flow to the c-axis oriented film. A new technology has been ardently expected for fabricating a-axis oriented films with which c-axis is parallel. The a-axis oriented film is defined as a film whose normal is parallel with a-axis. If an a-axis oriented film were produced, an a-axis junction would be obtained. The a-axis junction would exhibit excellent joint properties, since a-coherent length .xi..sub.a is long enough in comparison with c-coherent length .xi..sub.o. Nevertheless, nobody has succeeded in making an a-axis oriented film of good quality. All the a-axis oriented films which have been produced have a plenty of defects. The defects impede the current flow in the a-axis direction. The production of high quality a-axis films remains still a dream of researchers.
FIG. 1 is a graph showing the relation between the substrate temperature (.degree. C.) and the transition temperature Tc (K) when YBCO (YBa.sub.2 Cu.sub.3 O.sub.7-x) films are grown on SrTiO.sub.3 single crystal substrates described in Hidchumi Asano's thesis for a doctor's degree, "Research on the control of orientations of crystal growth of high temperature oxide superconductor films and the properties of same", p88 (1992). The abscissa denotes the temperature (.degree. C.) of the substrates. The ordinate is the transition temperatures Tc (K: Kelvin). The growths were done at two different oxygen partial pressures of 3.8 mTorr and 0.76 mTorr. Rounds designate the results at a 0.76 mTorr oxygen partial pressure. Squares denote the data at an oxygen partial pressure of 3.8 mTorr. An increase of the transition temperature Tc accompanies the rise of the substrate temperature between 480.degree. C. and 600.degree. C. In the same range, the transition temperature depends upon the partial pressure of oxygen of the atmosphere in the chamber. Blank rounds or blank squares denote the growth of a-axis oriented films. Black rounds or black squares denote the formation of c-axis oriented films. What is important is the fact that low temperature growth between 400.degree. C. and 580.degree. C. enables the SrTiO.sub.3 substrate to yield a-axis orientated films as shown by the assemblies of blank rounds and blank squares. Higher substrate temperature between 600.degree. C. and 680.degree. C. is likely to let the SrTiO.sub.3 substrate make c-axis oriented films. Mixtures of a-axis oriented parts and c-axis oriented parts are produced in the intermediate range between 580.degree. C. and 600.degree. C. The mixture films are denoted by dotted rounds or dotted squares.
Therefore the heteroepitaxy of a YBCO film on a SrTiO.sub.3 monocrystal substrate selectively produces either an a-axis oriented film or a c-axis oriented film by controlling the temperature of the substrate. The property by which the film orientation is determined by the temperature is hereinafter referred to as a temperature preference of orientation. The substrate has no crystallographical anisotropy between a-axis and c-axis, because it is a cubic crystal. But the YBCO crystal itself has a power of selecting orientations of growth. An a-axis oriented film can be made on the SrTiO.sub.3, crystal by the low temperature growth from 400.degree. C. to 580.degree. C. Then one may imagine an a-axis junction can be produced by piling a plurality of a-axis oriented films on a SrTiO.sub.3 substrate at the low temperature. However, this does not hold true. The low temperature grown films have plenty of defects. The defects hinder a big current from flowing in the a-axis oriented crystal in the superconductive state. Defective a-axis oriented films are useless, because the defects restrict the current despite the long coherent length in the normal direction. A-axis oriented films without defects are desired at present. One purpose of the invention is to provide a method of producing defect-free a-axis oriented films of oxide superconducting materials. Another purpose of the invention is to provide a method of making a-axis oriented films at high temperature. Another purpose of the invention is to provide a method of fabricating a junction which enables a large current to flow across the boundary.